(1) Field of the Invention
The present invention lies in the field of information exchange systems, and more particularly in the field of information exchange systems used in avionics systems that comply with aviation standards. The present invention relates to a chained information exchange system comprising a plurality of modules connected together by hardened digital buses.
(2) Description of Related Art
Present-day avionics information exchange systems serve to connect together the components making up the system, and to do so they make use of digital buses implemented using a variety of technologies. Such digital buses may be unidirectional or multidirectional, they may be point-to-point, single transmitter and multiple receiver, or indeed multipoint. Furthermore, such digital buses need to satisfy requirements that are specific to the field of aviation as specified in dedicated standards, e.g. in terms of withstanding electromagnetic waves.
Unidirectional buses are frequently encountered, i.e. buses in which an electric signal can travel in one direction only between the components that are connected together by such a bus. Conversely, multidirectional buses allow electric signals to travel in both directions between the components. There are two types of multidirectional bus, there are so-called “half-duplex” buses that enable an electric signal to travel in either direction along the bus, but in only one direction at a time, and there are also so-called “full-duplex” buses that allow electric signals to travel simultaneously in both directions along the bus.
It is necessary to have two unidirectional buses between two components in order to obtain information exchange that is equivalent to a “full-duplex” multidirectional bus.
There also exist single-transmitter multi-receiver digital buses capable of transmitting an electric signal solely from a single transmitter component to a plurality of receiver components. Conversely, a multipoint digital bus is capable of connecting together a plurality of components, each component being capable of being both a transmitter and a receiver. The capacities of the connections between a plurality of components, and consequently the exchange of information are therefore simpler to put into place with buses of this multipoint type.
Within electronic equipment, digital buses can be used for example to interconnect electronic chips over distances that are short, of the order of a few millimeters to a few centimeters. Generally, components in such equipment are protected, in particular against electromagnetic waves coming from external elements, by using the electronic equipment itself, e.g. by using a so-called “Faraday” cage.
Digital buses are also used for connecting together two such pieces of electronic equipment over distances that may lie in the range a few centimeters to several tens of meters. Under such circumstances, in order to be capable of avoiding or at least minimizing the appearance of noise in the electric signal traveling over such a digital bus, the digital buses must also be capable of withstanding the external environment that might generate electromagnetic disturbances. Such digital buses incorporate protection against such disturbances. In the description below, such digital buses are referred to as “hardened digital buses” or more simply as “hardened buses”.
By way of example, such electromagnetic disturbances may be generated by other wiring laid in the same bundle as the digital bus, by other pieces of electronic or electrical equipment close to the digital bus, and also by lightning. The protection of such hardened buses may be obtained merely by twisting together two electric wires, or else by shielding a bundle of electric wires and also by using filters on the interfaces with modules, these filters being constituted by one or more capacitors, together with inductors. Such hardened buses generally present impedance that is low, of the order of a few tens of ohms to a few kilohms, so as to be less sensitive to such electromagnetic disturbances.
The term “electronic equipment” is used to cover any peripheral or module that may be incorporated in an information exchange system and that is capable of receiving or of transmitting an electric signal. More particularly, the electronic equipment to which the invention applies comprises onboard modules, in particular modules on board aircraft, such as computers and actuators, for example.
The electric signal traveling along a digital bus conveys information that is exchanged between modules by being transferred over such digital buses. This information is encoded by bits capable of taking two values “1” and “0”. The information may be encoded on a single bit, corresponding to two states of the signal. The voltage of the electric signal then switches between a first value, for which the bit is equal to “1”, and a second value for which the bit is equal to “0”.
The information may also be encoded on two bits, thereby corresponding to three states of the electric signal. The voltage of the electric signal then varies between a high value corresponding to the “1” state, in which the first bit is equal to “1”, and the second bit is equal to “0”, and a low state, generally a negative state, corresponding to the state “0”, in which the first bit is equal to “0” and the second bit is equal to “1”. In a third state for which the voltage of the electric signal is zero, the first and second bits are both equal to “0”. This third state, corresponds to no information, but it may be used for example to detect a failure in the information exchange system. The two bits used for coding the signal may be referred to as “secondary signals”.
This form of coding using two secondary signals is used on certain buses, e.g. after input decoding of bus buffers in application of the ARINC 429 standard that is used in the field of aviation.
In the description below, the term “signal” is used on its own to designate the electric signal conveyed by the digital buses. Similarly, the term “bus” is used more simply for designating digital buses.
The buses used in avionics information exchange systems may comply with standards that are specific to the field of aviation, such as the MIL 1553 or AFDX standards, which define multidirectional multipoint buses, or indeed the ARINC 429 standard, which defines single-transmitter, multi-receiver buses that are unidirectional.
Buses in compliance with the MIL 1553 standard are mainly used in militry applications, and buses in compliance with the AFDX standard also known as the ARINC 664 standard, which are based on the “Ethernet” bus, are used in commercial aircraft.
Those standards also define the data rates that can be conveyed by buses complying with those standards. Such data rates are characterized by the maximum number of bits that can be transmitted per second. The term “bandwidth” is used for designating this maximum information rate that characterizes a bus.
For example, multidirectional buses in compliance with the MIL 1553 standard have a bandwidth of 1 megabit per second (Mbps) and those in compliance with the AFDX standard have a bandwidth of 10 Mbps or 100 Mbps. In contrast, single-transmitter, multi-receiver buses that are monodirectional in compliance with the ARINC 429 standard have narrower bandwidths, of 12.5 kilobits per second (kbps) or of 100 kbps.
It is also possible to use buses in compliance with the RS422 standard, these buses being single-transmitter, multi-receiver, unidirectional buses with a bandwidth of up to 10 Mbps.
The performance of a bus is defined by these bus characteristics, whether they refer to bandwidth or to the fact that a bus is multidirectional or else unidirectional and single-transmitter, multi-receiver, or indeed multipoint. Buses in compliance with the MIL 1553 and AFDX standards are multidirectional buses with large bandwidth, and they provide higher performance than buses in compliance with the ARINC 429 standard, but they are also more expensive.
Nevertheless, a multidirectional and multipoint bus makes it possible to connect together a plurality of components and allows signals to pass between any pair of components in either direction. Each component is thus capable of exchanging information with any other component with which it is connected, i.e. it is capable both of sending information and of receiving it.
However, in a single-transmitter, multi-receiver, unidirectional bus it is possible to connect only one signal-transmitting component to a plurality of other components that are capable only of receiving the signal, with the signal being capable of passing between the components in one direction only from the transmitter component to the receiver components. Furthermore, if it is desired to have a plurality of transmitter components, then the number of unidirectional buses increases correspondingly, as does the number of inputs needed on each component. Such a configuration using single-transmitter, multi-receiver, unidirectional buses quickly becomes very complex and expensive, in spite of the low cost of the individual buses used.
Furthermore, such buses are limited in practice to the number of receivers that can be connected thereto. For example, a bus in compliance with the ARINC 429 standard, which in theory can use up to twenty receivers, is generally limited to four or five receivers. The protection provided against electromagnetic disturbance on such buses, and in particular the capacitors that are connected thereto, can lead to distortion in the signals passing through them, and the distortion can become excessive when the number of receivers increases.
Another known bus complies with the controller area network (CAN) standard, which is inexpensive and in widespread use in the automotive industry. Nevertheless, few components dedicated to that bus are qualified at present with respect to aviation standards. Its use in information exchange systems in the field of aviation remains very limited.
Likewise, test means suitable for validating such systems making use of CAN buses are still not widespread. The cost of such test means needs to be taken into account in the overall cost of such information exchange systems. In contrast, such test means already exist for systems using common buses such as buses in compliance with the ARINC 429 standard.
There also exist solutions for limiting the number of connections within equipment made up of a plurality of components. In the field of microprocessor systems, for example, peripherals are connected together in order to form an open chain or a closed loop. Such a configuration is commonly referred to as a “daisy chain”.
Document US 2012/0166695 discloses an electronic system comprising a master module and a plurality of slave modules forming a daisy chain. A synchronization signal and a data signal are transmitted by the master module and they travel through each slave module before returning to the master module. The data signal transmitted by the master module may be of two types. In a first type of signal, the data signal contains common information for all of the slave modules, in which case none of the slave modules can modify the data signal. In a second type of signal, the data signal contains information addressed to a single slave module, which slave module can then modify the data signal in order to add information addressed solely to the master module. Each master or slave module may include a control unit such as a microprocessor, a microcontroller, or a field programmable gate array (FPGA), and these synchronizations and data signals may pass between the various modules over optical connections.
Document WO 2009/060153 describes avionics equipment for an aircraft that is capable of transmitting and receiving optical signals, together with an avionics system including at least one piece of such equipment. Such equipment includes an electro-optical connection interface capable of transforming both an optical signal into an electric signal and also an electric signal into an optical signal.
Finally, document EP 2 293 429 describes a secure monitoring and control device having an actuator control module and a monitoring module. The monitoring and control modules verify that the control signals and the signals from the sensors are consistent in order to control a module for powering the actuator.